1. Field of the Invention
This invention relates to a dry type isostatic pressing method, and more particularly to the improvement thereof characterized by supplying forcibly gas between the outer peripheral surface of a molded body or a shaping thin rubber bag where the molded body is housed and the inner peripheral surface of the shaping rubber mold, and the gas having a pressure higher than the isostatic pressure applied to the shaping rubber mold at the time when the said shaping rubber mold begins to restore to its original state during the decompression step thereof, as well as to an apparatus therefor.
2. Description of the Prior Art
In this kind of isostatic pressing, powders to be molded which have been packed in a rubber mold are compressed and molded in the presence of a isostatic pressure imparted to the rubber mold, and the isostatic pressing has often been utilized in the formation of pottery articles, fire bricks, ceramic moldings, etc. However, starting molded bodies for forming these articles have, in general, low tensile strength and low modulus of elasticity, and so some defects have been inevitable in the molding step thereof such that molded bodies are broken or cracks in the form of round slices occur in the molded bodies, when the molded bodies are released from the rubber mold during the decompression step, particularly in case of shaping long and slender columns.
Our experiments have clarified the following facts: In the molding operation according to the isostatic pressing, in general, the rubber mold in the apparatus is gradually compressed, and after the isostatic pressure has reached a desired one, the pressure is kept as such for a predetermined period of time, and thereafter the step transfers to the decompression step, while a non-compressible fluid such as oil, water or the like is used as a pressure medium, and so when the step has transferred to the decompression step, the rubber mold is restored to the original state thereof by the aid of the elastic force of the rubber mold itself at almost the termination of the reduced pressure step, without any rapid restoration corresponding to the rapid reduced pressure effect. In the restoration of the rubber mold, the restoring timing in the direction of the axis of the mold is earlier than that in the perpendicular direction thereof, in other words, the restoration of the rubber mold in the axis direction thereof is effected in such a state that a pressure is imparted to the molded body in the perpendicular direction of the axis thereof, and thus, a friction occurs between the surface of the rubber mold and that of the shaped molding and the resulting friction power on the surface of the molded body acts on the said surface as a tension. On these grounds, the molded body is broken or cracks occur therein. In addition, it also has been clarified that the more the shaping rubber mold thickens, the more the said tension increases.
In order to eliminate the said drawbacks, for example, such a means has heretofore been taken that powders to be molded are put in a thin rubber bag so as to evade the direct contact between the powders and the rubber mold, an antifriction agent is coated on the outer surface of the rubber bag. However, said means inevitably causes some drawbacks such that the operation step is complicated and the rubber mold is tainted or damaged by the coating of the antifriction agent.